the last guy slashed taxes too far, which created a structural deficit problem. it's the "starve the beast" thing. then, when there's deficits, he has to sell stuff and cut services. but, he went out and campaigned on tax cuts and balanced budgets. obvious contradiction, but we know that a lot of conservatives just aren't that bright. on top of that, oil just crashed, which has created a further revenue problem because he sent everybody else up the creek to create a petro dollar.
what trudeau (or the liberal party - trudeau is not a prime decision maker in the party, he really is mostly a good looking front person) did was look at the situation and realize that there was no possible way anything but a deficit could happen. yeah, it's true that harper was campaigning on a surplus, but he was just flat out lying about it.
faced with the reality of structural budget deficits, and a desire for infrastructure spending, they made the choice to approach the situation incrementally - which meant going into deficit in the short term to reverse this "starve the beast" stuff. that meant setting an expectation of the need for deficits, so they didn't get nailed on it for the next however many years. but what that means is that it was as much about painting the conservatives as fiscal imbeciles as it was about anything else.
the liberal party actually has a history and reputation in canada of superior fiscal management (much as the democrats do in the united states), and would have certainly preferred to campaign on balanced budgets. it's just that it was an impossible thing to follow through on, and they decided it was politically better to be transparent about it.